A ScrollController was used after being disposed.

[scalz]

Bug report

Describe the bug
Hello. Thank you for sharing your amazing work.

When providing a ScrollController to a SliverWoltModalSheetPage, when resizing window (tested on desktop/Windows), this error happens: "A ScrollController was used after being disposed."

Because I guess WoltModalSheetAnimatedSwitcher disposes _scrollControllers.

Am I doing something wrong, or is it a bug?

I'm using a workaround (I modified WoltModalSheetAnimatedSwitcher), in dispose(), by disposing only if scrollcontroller is "internal", and if external, removing its position listener (recreated by _subscribeToCurrentPageScrollPositionChanges).
For the moment it only works if I use a topbar (but this does the job for the moment as I'm using a topbar).
I'll try to dig in package to better understand how everything works, but you probably have a better idea how to fix it than me :)

Thank you in advance for your help.

Steps to reproduce

Steps to reproduce the behavior:

1. Reproducible example based on Basic Example from package

import 'package:flutter/material.dart';
import 'package:wolt_modal_sheet/wolt_modal_sheet.dart';

void main() {
  runApp(
    const WoltModalSheetApp(),
  );
}

class WoltModalSheetApp extends StatelessWidget {
  const WoltModalSheetApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: WoltModalSheetHomePage(),
    );
  }
}

class WoltModalSheetHomePage extends StatelessWidget {
  const WoltModalSheetHomePage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Wolt Modal Bottom Sheet Sample'),
      ),
      floatingActionButton: FloatingActionButton.extended(
        onPressed: () async {
          final scrollController = ScrollController();
          await WoltModalSheet.show(
            context: context,
            pageListBuilder: (bottomSheetContext) => [
              SliverWoltModalSheetPage(
                scrollController: scrollController,
                mainContentSliversBuilder: (context) => [
                  SliverList.builder(
                    itemBuilder: (context, index) {
                      return ListTile(
                        title: Text('Index is $index'),
                        onTap: Navigator.of(bottomSheetContext).pop,
                      );
                    },
                  ),
                ],
              )
            ],
          );
          scrollController.dispose();
        },
        label: const Text('Trigger Wolt Sheet'),
      ),
    );
  }
}

2. Run as Desktop app
3. Resize window to switch layouts between bottomsheet and dialog, and you should get an error.

Expected behavior

When providing an external ScrollController to a SliverWoltModalSheetPage, we should be responsible of disposing it.

---

Additional context

I stumbled on this error while I wanted to programmatically scroll to end of the list (for example when receiving an api event, adding an element + scrolling to end).

---

[ulusoyca]

Hi,

Thank you for reporting this bug, and sorry for being unresponsive due to holiday slow-down time. Would you have time to debug with me?

I suspect that the bug could be caused by this line:

  void _resetScrollControllers() {
    _scrollControllers.clear();
    _scrollControllers = [
      for (int i = 0; i < _pagesCount; i++)
        (_page.scrollController ??
            ScrollController(initialScrollOffset: _scrollPositions[i].value)),
    ];
  }

this should have been:

(widget.pages[i].scrollController ??
            ScrollController(initialScrollOffset: _scrollPositions[i].value)),

WDYT?

[scalz]

Hi.

No problem :)

I just tried your idea but it's not enough, controller is still getting disposed, in dispose() of WoltModalSheetAnimatedSwitcher.

The workaround I used for debugging:

Added a bool to _scrollControllers to keep track of which ones should be autodisposed by WoltModalSheetAnimatedSwitcher dispose().

List<({ScrollController c, bool autoDispose})> _scrollControllers = [];

ScrollController get _currentPageScrollController =>
    _scrollControllers[_pageIndex].c;
      
void _resetScrollControllers() {
  _scrollControllers.clear();
  _scrollControllers = [
    for (int i = 0; i < _pagesCount; i++)
      (c: (widget.pages[i].scrollController ??
          ScrollController(initialScrollOffset: _scrollPositions[i].value)),
      autoDispose: widget.pages[i].scrollController == null),
  ];
}

Created a method _currentPageScrollPositionChanges for adding and removing position Listener.
Dispose only scrollControllers which have autoDispose: true, and taking care of removing position Listener. Like this, custom scrollControllers don't get disposed.

  void _currentPageScrollPositionChanges() {
    if (_currentPageScrollController.hasClients) {
      _currentPageScrollPosition.value =
          _currentPageScrollController.position.pixels;
    }
  }

  void _subscribeToCurrentPageScrollPositionChanges() {
    for (final scrollController in _scrollControllers) {
      scrollController.c.addListener(_currentPageScrollPositionChanges);
    }
  }

  @override
  void dispose() {
    _animationController?.dispose();
    for (final element in _scrollControllers) {
      if (element.autoDispose) {
        element.c.dispose();
      }
      else element.c.removeListener(_currentPageScrollPositionChanges);
    }
    for (final element in _scrollPositions) {
      element.dispose();
    }
    super.dispose();
  }      

It works if I enable isTopBarLayerAlwaysVisible, which is fine for my case.
But if isTopBarLayerAlwaysVisible is disabled, then it triggers another error that I have not tried to fix yet.. snake bites its tail I guess.

ScrollController attached to multiple scroll views.
'package:flutter/src/widgets/scroll_controller.dart':
Failed assertion: line 172 pos 12: '_positions.length == 1'

The relevant error-causing widget was: 
  MaterialApp MaterialApp:file:///C:/projects/flutter/repro_wolt_scrollcontroller_issue/lib/main.dart:15:18
When the exception was thrown, this was the stack: 
#2      ScrollController.position (package:flutter/src/widgets/scroll_controller.dart:172:12)
#3      _TopBarFlowDelegate.currentScrollOffset (package:wolt_modal_sheet/src/content/components/main_content/wolt_modal_sheet_top_bar_flow.dart:81:54)
#4      _TopBarFlowDelegate.paintChildren (package:wolt_modal_sheet/src/content/components/main_content/wolt_modal_sheet_top_bar_flow.dart:98:11)```